The NIH halted the study after researchers realized that Prempro raised the risks of breast cancer, heart attack and stroke among women using the drug. The use of Prempro and other similar drugs fell sharply that year over concerns about the link between HRT and breast cancer or other side effects.

The strongest evidence of the risks of hormone replacement therapy came in a study by the University of Texas M.D. Anderson Cancer Center. Researchers found that new breast cancer cases fell by 7% nationwide in 2003, the year after many women had stopped using the drugs. After analyzing the potential reasons for this decline, the researchers concluded that the change could only be linked to the decline in women receiving hormone replacement therapy treatment.

12–15% drop in breast cancer for older women
While the 7% drop was “astounding” in the words of one scientist, the change was even more noticeable in the type of cancer associated with hormone replacement therapy and in the age group that usually receives this treatment. Among women 50–69 who receive HRT drugs during menopause, cancer rates fell by 12%. And for estrogen-positive tumors—which are fueled by the hormones in HRT—cancer rates fell by 15%.

Scientists say these numbers suggest that the decline in hormone replacement therapy use is the only probable cause of the falling cancer rates. About one-third of women over age 50 were using Prempro and other HRT drugs before 2002; about two-thirds of these women stopped using hormone replacement therapy in 2003 alone.

Doctors and scientists cautious but impressed with study results
Dr. Peter Ravdin, one of the authors of the M.D. Anderson study, says that this was the largest decline for a single cancer that he had ever seen. In all, about 14,000 fewer women were diagnosed with breast cancer in 2003, meaning that thousands of lives were potentially saved because of the decreased use of hormone replacement therapy.

The scientists in the M.D. Anderson study caution that the decline in breast cancer cases does not definitively prove a link with the decline in the number of women receiving hormone replacement therapy treatment. But they say that the timeline of events and the rapid drop in new cases after the treatment decline confirms that hormone replacement therapy was probably the cause.